I accidently paid the same invoice twice last month.its a merchant i use regularly so wasnt bothered.i need to show that overpayment as a credit on their account.

Hi grahamtheplumber, thanks for posting on the Community. 

You can record the overpayment easily by selecting 'Make payment' on the bill (invoice) and entering the full amount paid in the 'Amount' field. Once saved, this will create an unapplied payment on the merchants account of the difference between the bill amount and the bill payment amount. This can then be applied as the payment on future bills to the same merchant (by selecting 'Make payment' on the bill > tick the unapplied payment under 'Credits' to apply).